The invention generally relates to plethysmography which is the measurement of the size of an organ or limb and in the amount of blood flowing therein. More particularly, the invention relates to impedance plethysmography and the synonymous terms of bioimpedance, bioelectrical impedance, and impedance rheometry in which a pair of transmitting ring electrodes are used to apply a high frequency electric current to a patient and pair of readout ring electrodes are used to receive the varying high frequency electric current flowing through the part of the patient located between the transmitting ring electrodes and the readout ring electrodes associated therewith. The current flow between the transmitting and associated ring electrodes is a function of the variation of the electrical impedance therebetween. The electrical impedance in turn is a function of the variation of blood flow through the part of the patient encircled by the transmitting and receiving ring electrodes.